Use Machine/Deep Learning to Guess a String
As Andreas has commented this is a problem of statistical language model (a probability distribution over a sequence of words). The important thing you need is a hash table mapping fixed-length to the expected ending chains of words in your dictionary.
Things that can make your prediction better:


Add better and more words to your dictionary.
Use text expansion.


What you are looking for will require a pinch of Reinforcement Learning too. You need to figure out a way to penalize and award the predictions and then use the result in future. Your case also requires you to build your own corpus, which is the hardest part. If your corpus is good, it will give better results.
This is the research paper that will help you a lot.